Engine-induced aircraft cabin resonance reduction system and method

ABSTRACT

A system and method of reducing engine induced aircraft cabin resonance includes sensing the core engine speed of a first turbofan gas turbine engine, and sensing the core engine speed of a second turbofan gas turbine engine. In a control system, the core engine speed of the first turbofan gas turbine engine and the core engine speed of the second turbofan gas turbine engine are processed to determine a core engine speed difference between the first and second turbofan gas turbine engines. The core engine speed difference is processed to supply a variable inlet guide vane (VIGV) offset value. The VIGV offset value is applied to a VIGV reference command associated with one of the first or second turbofan gas turbine engine to thereby cause the VIGVs of one of the first or second turbofan gas turbine engine to move to a more closed position.

TECHNICAL FIELD

The present invention generally relates to aircraft cabin resonance, andmore particularly relates to a system and method for reducingengine-induced aircraft cabin resonance.

BACKGROUND

On some multi-engine aircraft, engine-induced acoustic resonance can begenerated, resulting in noise and sometimes vibration being transmittedinto the aircraft cabin. The resulting noise and vibration can lead topassenger discomfort. Known approaches to controlling noise andvibration include matching the rotational speeds and phase relationshipsof the aircraft engines. In many of these known approaches, one engineis selected as a “master” engine and the other as a “slave” engine. Whena speed mismatch of sufficient magnitude occurs, the engine speed of the“slave” engine is adjusted to equal that of the master engine.

In the case of multi-spool engines, such as turbofan, turboprop orprop-fan gas turbine engines, only a single spool is typicallysynchronized. Typically, the speeds of the fan spool (N1) aresynchronized. However, the speeds of the high pressure spools (N2)remain non-synchronized, resulting in continued generation ofundesirable noise and/or vibration. Attempts have been made tosynchronize both N1 and N2 speeds in multi-spool engines. Some currentapproaches rely on closed-loop control of N2 speed based on sensed N2speed and or one or more other engine parameters. These approaches canexhibit stability issues. Another approach has been to attempt toprecision balance the engines prior to installation. This approach canbe time-consuming and costly.

Hence, there is a need for reducing engine-induced aircraft cabinresonance that does not rely on closed-loop N2 control and/or does notrely on precision engine balancing. The present invention addresses atleast this need.

BRIEF SUMMARY

This summary is provided to describe select concepts in a simplifiedform that are further described in the Detailed Description. Thissummary is not intended to identify key or essential features of theclaimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In one embodiment, a method of reducing engine induced aircraft cabinresonance includes sensing the core engine speed of a first turbofan gasturbine engine, and sensing the core engine speed of a second turbofangas turbine engine. In a control system, the core engine speed of thefirst turbofan gas turbine engine and the core engine speed of thesecond turbofan gas turbine engine are processed to determine a coreengine speed difference between the first and second turbofan gasturbine engines. The core engine speed difference is processed to supplya variable inlet guide vane (VIGV) offset value. The VIGV offset valueis applied to a VIGV reference command associated with one of the firstor second turbofan gas turbine engine to thereby cause the VIGVs of oneof the first or second turbofan gas turbine engine to move to a moreclosed position.

In another embodiment, an engine induced aircraft cabin resonancereduction system includes a first speed sensor, a second speed sensor,and a control system. The first speed sensor is configured to sense coreengine speed of a first turbofan gas turbine engine and supply a firstcore engine speed signal indicative thereof. The second speed sensor isconfigured to sense core engine speed of a second turbofan gas turbineengine and supply a second core engine speed signal indicative thereof.The control system is coupled to receive the first and second coreengine speed signals and is configured, upon receipt thereof, todetermine a core engine speed difference between the first and secondturbofan gas turbine engines, determine a variable inlet guide vane(VIGV) offset value based, at least in part, on the core engine speeddifference, and apply the VIGV offset value to a VIGV reference commandassociated with one of the first or second turbofan gas turbine engines,to thereby command the VIGVs of one of the first or second turbofan gasturbine engine to move to a more closed position.

In yet another embodiment, an engine induced aircraft cabin resonancereduction system includes a first speed sensor, a second speed sensor,and a control system. The first speed sensor is configured to sense coreengine speed of a first turbofan gas turbine engine and supply a firstcore engine speed signal indicative thereof. The second speed sensor isconfigured to sense core engine speed of a second turbofan gas turbineengine and supply a second core engine speed signal indicative thereof.The control system is coupled to receive the first and second coreengine speed signals and an enable signal indicating that fan speedsynchronization of the first and second turbofan gas turbine engines hasbeen enabled. The control system is configured, upon receipt of thefirst and second core engine speed signals and the enable signal, to setthe turbofan gas turbine engine having a lower core engine speed as aslave turbofan gas turbine engine, set the turbofan gas turbine enginehaving a higher core engine speed as a master turbofan gas turbineengine, determine a core engine speed difference between the slave andmaster turbofan gas turbine engines, determine a variable inlet guidevane (VIGV) offset value based, at least in part, on the core enginespeed difference, and apply the VIGV offset value to a VIGV referencecommand associated with the slave turbofan gas turbine engine, tothereby command the VIGVs of the slave turbofan gas turbine engine tomove to a more closed position.

Furthermore, other desirable features and characteristics of the engineinduced aircraft cabin resonance reduction system and method will becomeapparent from the subsequent detailed description and the appendedclaims, taken in conjunction with the accompanying drawings and thepreceding background.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and wherein:

FIG. 1 depicts a functional block diagram of one embodiment of anexemplary engine control system for an aircraft;

FIG. 2 depicts an embodiment, in flowchart form, of an exemplaryopen-loop N2 synchronization control process that may be implemented inthe system of FIG. 1;

FIG. 3 depicts a functional block diagram of logic that may beimplemented in the system of FIG. 1 to carry out the process depicted inFIG. 2.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. As used herein, the word “exemplary” means “serving as anexample, instance, or illustration.” Thus, any embodiment describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments. All of the embodiments describedherein are exemplary embodiments provided to enable persons skilled inthe art to make or use the invention and not to limit the scope of theinvention which is defined by the claims. Furthermore, there is nointention to be bound by any expressed or implied theory presented inthe preceding technical field, background, brief summary, or thefollowing detailed description.

Turning now to FIG. 1, a functional block diagram of an exemplary enginecontrol system 100 for an aircraft is depicted and includes a pair ofturbofan gas turbine engines 102—a first turbofan gas turbine engine102-1 and a second turbofan gas turbine engine 102-2—and a controlsystem 104. The turbofan gas turbine engines 102 are both implemented asmulti-spool gas turbine engines and, as FIG. 1 further depicts, eachincludes an intake section 106, a compressor section 108, a combustionsection 112, a propulsion turbine 114, and an exhaust section 116. Theintake section 106 includes a fan 118, which draws air into the intakesection 106 and accelerates it. A fraction of the accelerated airexhausted from the fan 118 is directed through a bypass section 119disposed between a fan case 121 and an engine cowl 123, and provides aforward thrust. The remaining fraction of air exhausted from the fan 118is directed into the compressor section 108.

The compressor section 108, which may include one or more compressors,raises the pressure of the air directed into it from the fan 118, anddirects the compressed air into the combustion section 112. In thedepicted embodiment, only a single compressor 110 is shown, though itwill be appreciated that one or more additional compressors could beused. In the combustion section 112, which includes a combustor assembly113, the compressed air is mixed with fuel supplied from anon-illustrated fuel source. The fuel and air mixture is combusted, andthe high energy combusted air mixture is then directed into thepropulsion turbine 114.

The propulsion turbine 114 includes one or more turbines. In thedepicted embodiment, the propulsion turbine 114 includes two turbines, ahigh pressure turbine 122, and a low pressure turbine 124. However, itwill be appreciated that the propulsion turbine 114 could be implementedwith more or less than this number of turbines. No matter the particularnumber, the combusted air mixture from the combustion section 112expands through each turbine 122, 124, causing it to rotate. Thecombusted air mixture is then exhausted through the exhaust section 116providing additional forward thrust.

As the turbines 122 and 124 rotate, each drives equipment in the engine102 via concentrically disposed shafts or spools. Specifically, the highpressure turbine 122 drives the compressor 110, via a high pressurespool 126, at a rotational speed that is generally referred to as coreengine speed (N2). The low pressure turbine 124 drives the fan 118, viaa low pressure spool 128, at a rotational speed that is generallyreferred to as fan speed (N1).

Each turbofan gas turbine engine 102 also includes variable inlet guidevanes (VIGVs) 132 and a plurality of sensors 134. The VIGVs 132, as isgenerally known, are disposed in the compressor section 108 of theengine 102 and are used to control the amount of airflow into andthrough the compressor section 108, and thus into and through thepropulsion turbine 114. It is also generally known that the positions ofthe VIGVs 132 impact the core engine speed (N2). Specifically, movingthe VIGVs 132 to a more closed position will cause core engine speed toincrease, and vice-versa.

The sensors 134 are used to supply data representative of engineoperating conditions and engine flight conditions. It will beappreciated that the number and specific types of data supplied by thesensors 134 may vary. In the depicted embodiment, however, at least asubset of the depicted sensors 134 are configured to supply datarepresentative of, or that can be used to determine, corrected fan speed(N1/θ^(0.5)), core engine speed (N2), current flight conditions, andcurrent engine operating conditions. It will be additionally appreciatedthat the particular number, type, and location of each of the sensors134 that are used to supply these data may vary. In the depictedembodiment, however, the sensors 134 that are used include first andsecond fan speed sensors, first and second core engine speed sensors,first and second total inlet pressure sensors, and first and secondtotal inlet temperature sensors.

The first and second fan speed sensors are configured to sense the fanspeed of the first and second turbofan gas turbine engines 102,respectively, and supply first and second core engine speed signalsindicative thereof, respectively. The first and second core engine speedsensors are configured to sense the core engine speed of the first andsecond turbofan gas turbine engines 102, respectively, and supply firstand second core engine speed signals indicative thereof, respectively.The total inlet pressure sensors are configured to sense the totalpressures at the engine inlets and supply signals representativethereof. As is generally known, sensed total inlet pressure isrepresentative of current aircraft altitude and airspeed. The totalinlet temperature sensors are configured to sense the inlet temperaturesat the engine inlets and supply signals representative thereof. As isgenerally known, sensed total inlet temperature is used to derive thecorrection factor (θ), which is used to derive corrected fan speed(N1/θ^(0.5)). It will be appreciated that total inlet pressure and/ortotal inlet temperature may be sensed at the engine inlets or derivedfrom measurements elsewhere on the aircraft or engines.

The control system 104 is in operable communication with, and isconfigured to control the operation of the engines 102. In the depictedembodiment, the control system 104 includes a pair of engine controls136—a first engine control 136-1 and a second engine control 136-2—thatare in operable communication with each other. Each engine control 136is also in operable communication with one of the engines 102 and isconfigured, in response to a throttle setting 138, to control the flowof fuel to, and thus the power generated by, that engine 102. Moreover,and as will be now described, the engine controls 136 work together toselectively implement a method of reducing engine induced aircraft cabinresonance by implementing an open-loop N2 synchronization controlprocess.

An embodiment of the open-loop N2 synchronization control process thatthe control system 104 implements is depicted, in flowchart form, inFIG. 2. This process 200, together with a functional block diagram ofthe logic 300 implemented in the control system 104 to carry out thisprocess 200, will now be described. Before doing so, it is noted thatthe process 200 may be initiated automatically or, as FIG. 1 depicts, inresponse to an input (N2 SYNC) to the control system 104. Whether it isinitiated manually or automatically, the process 200 is also preferablyinitiated only after certain conditions are met. These conditions mayvary, but in the depicted embodiment include the aircraft being within apredetermined altitude range, the throttle setting being with apredetermined throttle setting range, the engines 102 being in arelatively steady state condition, meaning not accelerating ordecelerating and not supplying bleed air to major bleed loads such asaircraft anti-ice, and the fan speeds (N1) of the engines 102 beingsynchronized.

The process 200, when initiated, begins with sensing the core enginespeed of the first and second turbofan gas turbine engines 102 (202).The core engine speeds of the first and second turbofan gas turbineengines (N2 ₁, N2 ₂) are then processed, in the control system 104, todetermine a core engine speed difference (ΔN2) (204). Though notdepicted in FIG. 2, it is noted that in a preferred embodiment, thecontrol system 104 first processes the core engine speeds (N2 ₁, N2 ₂)of the first and second turbofan gas turbine engines 102 to determinewhich has a lower core engine speed. The engine 102 with the lower coreengine speed is set as the slave engine, and the engine with the highercore engine speed is set as the master engine. As depicted in FIG. 3,the control system 104 implements a subtraction function 302 todetermine the core engine speed difference (ΔN2).

After the core engine speed difference (ΔN2) is determined, the controlsystem 104 processes the core engine speed difference (ΔN2) to supply aVIGV offset value (dVIGV) (206). The control system 104 may be variouslyconfigured to implement this function, but in the embodiment depicted inFIG. 3, it uses a look-up table 304 and a multiplier function 306. Inparticular, the look-up table 304 includes a plurality of pre-storedVIGV sensitivity values (dN2/dVIGV). Each VIGV sensitivity value(dN2/dVIGV) is representative of an incremental change in the coreengine speed (N2) of the slave engine with an incremental change in theVIGV position at all aircraft flight conditions and engine powerconditions of interest. The VIGV sensitivity values (dN2/dVIGV) aregenerated using a calibrated engine model, from engine testing, or both.Specifically, at the various aircraft flight conditions and engine powerconditions of interest, and with a constant fan air speed (N1), thenormal VIGV command is incrementally varied by an offset value (e.g.,dVIGV). The incremental variation in the normal VIGV command will resultin an incremental change in core engine speed (e.g., dN2).

It was noted above that the stored VIGV sensitivity values (dN2/dVIGV)are generated at aircraft flight conditions and engine power conditionsof interest. These conditions may vary, but are typically conditionsrepresentative of cruise numbers. That is, aircraft altitude and enginespeeds associated with cruise conditions. Thus, as FIG. 3 depicts,parameters representative of current aircraft flight conditions andcurrent engine power setting are inputs to the look-up table 304.Although these specific parameters may vary, in the depicted embodimentthese parameters are total inlet pressure (PTO) and corrected fan speed(N1/θ^(0.5)).

Regardless of the specific parameters that are used to select the VIGVsensitivity value (dN2/dVIGV) in the look-up table, the inverse of theselected VIGV sensitivity value (e.g., dVIGV/dN2) is supplied to themultiplier function 306. As FIG. 3 also depicts, the core engine speeddifference (ΔN2) is also supplied to the multiplier function 306. Themultiplier function 306, using these two values as operands, generatesthe VIGV offset value as follows:dVIGV=(ΔN2)×(dVIGV/dN2).

Returning momentarily to FIG. 2, it is seen that after the VIGV offsetvalue (dVIGV) is supplied, the control system 104 then applies the VIGVoffset value (208). In particular, as FIG. 3 depicts, the VGIV offsetvalue is applied to a VIGV reference command associated with one of theturbofan gas turbine engines 102, and most preferably the previouslyidentified slave engine 102. To do so, the control system 104 implementsan addition function 308, which adds the VIGV offset value (dVIGV) tothe VIGV reference command associated with the slave turbofan gasturbine engine 102. The control system 104 at least in the depictedembodiment, additionally includes a limiter function 312. The limiterfunction 312, if included, is configured to limit the VIGV offset value(dVIGV) to a predetermined VIGV maximum offset value (dVIGV_(LIMIT)).

Regardless of whether the VIGV offset value is limited, when it isapplied it causes the VIGVs 132 of that turbofan gas turbine engine 102(e.g., the slave engine) to move to a more closed position. As a result,and as was previously mentioned, this will cause the core engine speed(N2) of that turbofan gas turbine engine 102 (e.g., the slave engine) toincrease an amount equivalent to, or at least substantially equivalentto, the core engine speed difference (ΔN2). Which means the core enginespeed (N2) of that turbofan gas turbine engine 102 (e.g., the slaveengine) will match, or at least come sufficiently close to matching, thecore engine speed (N2) of the other turbofan gas turbine engine 102(e.g., the master engine).

Referring once again to FIG. 2, it is seen that, after the VIGV offsetvalue is applied, the control system 104 determines whether or not apredetermined condition is met (212). If the predetermined condition isnot met, the same VIGV offset value continues to be applied. If thepredetermined condition is met, the process 200 repeats to establish andapply an updated VIGV offset value. It will be appreciated that thepredetermined condition may vary. But in the depicted embodiment thepredetermined condition is when the absolute value of the core enginespeed difference (ΔN2) exceeds a predetermined magnitude.

To implement the above-described function, the control system 104, asdepicted in FIG. 3, includes what is referred to herein as latchinglogic 314. This logic 314 includes a less-than function 316, and anif-then function 318. In particular, the less-than function 316 outputsa logical “1” when the absolute value of the core engine speeddifference is less than a predetermined core engine speed differencevalue (ΔN2 _(LIMIT)), otherwise it outputs a logical “0.” The if-thenfunction 318 uses the output of the less-than function 316 to set theVIGV offset value (dVIGV). Specifically, if the output of the less-thanfunction 316 is a logical “1,” then the VIGV offset value (dVIGV) is setto the output of the limiter function 312. Conversely, if the output ofthe less-than function 316 is a logical “0,” then the VIGV offset value(dVIGV) is set to zero, and an updated VIGV offset value is established.

The system and method described herein provide an open-loop controlstrategy to drive the N2 of the slave engine toward the N2 of the masterengine. The open-loop control strategy provides advantages overclosed-loop strategies, which can, at times, exhibit stability issues.

Those of skill in the art will appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Some ofthe embodiments and implementations are described above in terms offunctional and/or logical block components (or modules) and variousprocessing steps. However, it should be appreciated that such blockcomponents (or modules) may be realized by any number of hardware,software, and/or firmware components configured to perform the specifiedfunctions. To clearly illustrate this interchangeability of hardware andsoftware, various illustrative components, blocks, modules, circuits,and steps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention. For example, anembodiment of a system or a component may employ various integratedcircuit components, e.g., memory elements, digital signal processingelements, logic elements, look-up tables, or the like, which may carryout a variety of functions under the control of one or moremicroprocessors or other control devices. In addition, those skilled inthe art will appreciate that embodiments described herein are merelyexemplary implementations.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC. The ASIC may reside in a user terminal. In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal

In this document, relational terms such as first and second, and thelike may be used solely to distinguish one entity or action from anotherentity or action without necessarily requiring or implying any actualsuch relationship or order between such entities or actions. Numericalordinals such as “first,” “second,” “third,” etc. simply denotedifferent singles of a plurality and do not imply any order or sequenceunless specifically defined by the claim language. The sequence of thetext in any of the claims does not imply that process steps must beperformed in a temporal or logical order according to such sequenceunless it is specifically defined by the language of the claim. Theprocess steps may be interchanged in any order without departing fromthe scope of the invention as long as such an interchange does notcontradict the claim language and is not logically nonsensical.

Furthermore, depending on the context, words such as “connect” or“coupled to” used in describing a relationship between differentelements do not imply that a direct physical connection must be madebetween these elements. For example, two elements may be connected toeach other physically, electronically, logically, or in any othermanner, through one or more additional elements.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention. It being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:
 1. An engine induced aircraft cabin resonancereduction system, comprising: a first speed sensor configured to sensecore engine speed of a first turbofan gas turbine engine and supply afirst core engine speed signal indicative thereof; a second speed sensorconfigured to sense core engine speed of a second turbofan gas turbineengine and supply a second core engine speed signal indicative thereof;and a control system coupled to receive the first and second core enginespeed signals and a signal indicating that fan speed synchronization ofthe first and second turbofan gas turbine engines has been enabled, thecontrol system configured, upon receipt of these signals, to: determinewhich turbofan gas turbine engine has a lower core engine speed,determine a core engine speed difference between the first and secondturbofan gas turbine engines, determine a variable inlet guide vane(VIGV) offset value based, at least in part, on the core engine speeddifference, and apply the VIGV offset value to a VIGV reference commandassociated with the turbofan gas turbine engine having the lower coreengine speed, to thereby command the VIGVs of the turbofan gas turbineengine having the lower core engine speed to move to a more closedposition.
 2. The system of claim 1, further comprising: a flightcondition sensor configured to sense a parameter representative ofcurrent aircraft flight conditions and supply a flight condition signalindicative thereof; one or more engine operating condition sensorsconfigured to sense one or more parameters representative of currentengine operating conditions and supply an engine operating conditionsignal indicative thereof, wherein the control system is further coupledto receive the flight condition signal and the engine operatingcondition signal, and is further configured, upon receipt of thesesignals, to: supply a VIGV sensitivity value based on the currentaircraft flight conditions and the one or more engine operatingconditions, the VIGV sensitivity value representative of a variation incore engine speed difference with a change in VIGV position at thecurrent aircraft flight conditions, and determine the VIGV offset valuebased on the core engine speed difference and the VIGV sensitivityvalue.
 3. The system of claim 2, wherein: the control system furthercomprises a look-up table of predetermined VIGV sensitivity values; andthe control system selects the VIGV sensitivity value from the look-uptable, based on the current flight conditions and current engineoperating conditions.
 4. The system of claim 2, wherein: the parameterrepresentative of current aircraft flight conditions comprises totalinlet pressure sensor; and the one or more parameters representative ofcurrent engine operating conditions comprise corrected fan speed.
 5. Thesystem of claim 1, wherein the control system is further configured to:compare the core engine speed difference to a predetermined magnitude;and when the core engine speed difference has exceeded the predeterminedmagnitude: determine an updated variable inlet guide vane (VIGV) offsetvalue based, at least in part, on the core engine speed difference, andapply the updated VIGV offset value to the VIGV reference command. 6.The system of claim 1, wherein the control system is further configuredto limit the VIGV offset value to a predetermined VIGV maximum offsetvalue.
 7. An engine induced aircraft cabin resonance reduction system,comprising: a first speed sensor configured to sense core engine speedof a first turbofan gas turbine engine and supply a first core enginespeed signal indicative thereof; a second speed sensor configured tosense core engine speed of a second turbofan gas turbine engine andsupply a second core engine speed signal indicative thereof; and acontrol system coupled to receive (i) the first and second core enginespeed signals and (ii) an enable signal indicating that fan speedsynchronization of the first and second turbofan gas turbine engines hasbeen enabled, the control system configured, upon receipt of the firstand second core engine speed signals and the enable signal, to: set theturbofan gas turbine engine having a lower core engine speed as a slaveturbofan gas turbine engine, set the turbofan gas turbine engine havinga higher core engine speed as a master turbofan gas turbine engine,determine a core engine speed difference between the slave and masterturbofan gas turbine engines, determine a variable inlet guide vane(VIGV) offset value based, at least in part, on the core engine speeddifference, and apply the VIGV offset value to a VIGV reference commandassociated with the slave turbofan gas turbine engine, to therebycommand the VIGVs of the slave turbofan gas turbine engine to move to amore closed position.
 8. The system of claim 7, further comprising: aflight condition sensor configured to sense a parameter representativeof current aircraft flight conditions and supply a flight conditionsignal indicative thereof; one or more engine operating conditionsensors configured to sense one or more parameters representative ofcurrent engine operating conditions and supply an engine operatingcondition signal indicative thereof, wherein the control system isfurther coupled to receive the flight condition signal and the engineoperating condition signal, and is further configured, upon receipt ofthese signals, to: supply a VIGV sensitivity value based on the currentaircraft flight conditions, the VIGV sensitivity value representative ofa variation in core engine speed difference with a change in VIGVposition at the current aircraft flight conditions, and determine theVIGV offset value based on the core engine speed difference and thesensitivity value.
 9. The system of claim 8, wherein: the control systemfurther comprises a look-up table of predetermined VIGV sensitivityvalues; and the control system selects the VIGV sensitivity value fromthe look-up table, based on the current flight conditions and currentengine operating conditions.
 10. The system of claim 8, wherein theparameters representative of flight conditions comprise: a total inletpressure sensor; and a corrected fan speed.
 11. The system of claim 7,wherein the control system is further configured to: compare the coreengine speed difference to a predetermined magnitude; and when the coreengine speed difference has exceeded the predetermined magnitudedetermine an updated variable inlet guide vane (VIGV) offset valuebased, at least in part, on the core engine speed difference, and applythe updated VIGV offset value to the VIGV reference command.
 12. Thesystem of claim 7, wherein the control system is further configured tolimit the VIGV offset value to a predetermined VIGV maximum offsetvalue.